Loud speaker



1932- F; B. FLANDERS YETAL- 1,841,101

LOUD SPEAKER Filed March 6, 1930 3 i i x I FBJZAA/mms wvtwmfis H C HARRISON ATTORNEY Patented Jan. 12, 1932 UNITED STATES PATENT {OFFICE PAUL B. FLANDERS, OF EAST ORANGE, NEW JERSEY, AND HENRY C. HARRISON, 01 PORT WASHINGTON, NEW YORK, ASSIGNORS TO BELL TELEPHONE LABORATORIES,

INCORPORATED, OF NEW YORK, N. Y., A"CORPORATION OF NEW.YORK

LOUD 'srnaxnn Application filed March a,

This invention relates to loud speakers of the direct radiating diaphragm type and more particularly to mountings for inertia controlled speakers having piston dia- 5 phragms of the cone or stiff plate type.

These diaphragms as used, for instance, in moving coil loud speakers, are usually of comparatively small dimensions and must be provided with some form of baffle arrangement or its equivalent. If this is not done the response particularly at low frequencies, will be unsatisfactory due to the cancellation effects between the air waves set up by the opposite sides of the diaphragm These :3 speakers, therefore, were atone time usually mounted in an opening in a large lane surface which'acted as a baille boar between the front and-rear waves. The diaphragm was supported at its periphery by a mounting -member of low elasticity which gave it a low natural period and since the radiation resistance of the air is also low at low frequencies, this construction gave a. large low requency peak in the response curve and was,

tice to mount such cones in a cabinet or box of moderate size. open at the rear side. While it is possible in this way to reduce the cancellation effect materially, it is found that'this construction has the decided disadvantage of producing other irregularities in the frequency res onse due to reflection from the walls of the ox. Some of these reflected waves return to the diaphragm out of phase with this motion, thereby retarding it, while others are in phase thereby increasing the amplitude of its natural movement and producing the so-called barrel tone or resonant effect. It has been proposed to reduce these irregularities by lining the box therefore, not entirely satisfactory even aside dissipate this excess energy. an overlapping or rever er 1930. Serial No. 433,590.

done, the air confined within the box acts as an elastic reactance tending to retard the diaphragm and since this effect is inversely roportional to frequency the response is und uIy decreased at low frequencies.

r The variation of the radiation resistance of the air with frequency'has alread been mentioned. It is well-known that, whilethis resistance is comparatively constant for all sound frequencies having wave lengths greater than one-third the diameter of the diaphragm to be used, it falls off very rapidly for frequencies having wave len hs greater than this diameter. For the stea y state condition of vibration this is not ob ectionable.

The variation of velocit of vibration with frequencyis such that 2hr constant energy input the'sound output, (VR) is also substantially constant over the range of interest.

It will be evident however, that while a low frequency vibration is being built up in the diaphragm, the energy stored in the diaphragm 1 (yaw) is relatively large compared with the energy radiated (V R) so that the diaphragm must continue to Vibrate for an appreciable time. after the driving force has ceased in order to This results in ation efiect similar to that obtained in an excessively live room. Since the impedance of diaphragms of the type under discussion is almost entirely mass reactance, considerable resistance damping can be introduced to suppress these transients without materially increasing the total impedance of the system, that is, without substantially reducing the response of the diaphragm.

It is therefore the object of the present invention to so design the air chamber of such a loud speaker as to obtain a satisfactory baflie effect and at thesame time to reduce the transient period to a point where it is no longer objectionable.

One Very advantageous way of attaining these objects in acordance with the invention consists in using a cabinet of only moderate dimensions and enclosing the normally open side with a material which offers to the air flow an impedance of the nature of a pure acoustic resistance of-the proper magnitude. In this wa the cone is damped sufficiently to reduce t 1e transient effect materiallyand the air waves which pass through the membrane are so reduced in magnitude and shifted in phase that the result as regards cancellation is the same as if a large plane bafiie were used. It is found for instance that very satisfactory results may be obtained without materially reducing the efliciency of the loud speaker if the magnitude of this added resistance is correlated to the dimensions of the box in such a way that somewhat less than two thirds of the air displaced by the rear side of the cone passes through the encloslno' material.

This general statement of the invention will be more clearly understood from a consideration of the following description of a specific case in connection with the drawing in which Fig. 1 is a cross-section of a cone type loud speaker mounted in a baflie box with the back resistance added in accordance with the present invention; Fig. 2 is a detail of the acoustic resistance material employed; Fig. 3 is a representation of the analogs of the mechanical elements of the loud speaker of Fig. 1 in the elements of a corresponding electrical network; and Fig. 4 is a vector representation of the air velocities set up by the arrangement of Fig. 1.

For purpose of illustration assume an 18 cubical box 1 as shown in Fig. 1 with an 8" diameter cone diaphragm 2, mounted in the front side 3 of the box and adapted to be driven by the driving unit 4,-and an acoustic resistance material 5 covering the rear side. One form of such a resistance consists of a thin sheet of metal containing narrow slits 6 such as those described in French Patent 665,237 ranted to P. B. Flanders May 6, 1929. This system like any othermechanical system may be considered for the sake of convenience as a mechanical transmission line as indicated in Fig. 8 in which F is the driving force in dynes applied to the cone by the unit 1,717, is the mass of the cone in grams, 1-, is theradiation resistance due to the air on the front side of the cone, S is the stiffness of the air enclosed in the box, 7' is the radiation resistance at the back of the cone and R is the added resistance of the material 5. The velocities imparted to the air at the front and rear sides of the cone are represented by i and 77 respectively as indicated. Under these conditions the actual sound emitted is proportional to the vector sum. of i and i Obviously the ideal condition would be to have 2': alone effective.

From the analogy to an electrical circuit it follows that the impedance in Fig. 3 of S and R in parallel (neglecting 1}, which is comparatively small at low frequencies) is R jX and also that as regards cancellation effects in front of the cone The relation of the ideal to the actual condition is therefor expressed by The value of X, the stiffness reactance of the enclosed air, at any frequency such as cycles for which all of the constants of Fig. 3 are computed, is given by where S is the stiffness of the enclosed air and w is equal to 21rf.

p 2A12 S V3 l.4 1O 324 =4l 1 5 mechanical ohms.

(A mechanical ohm is defined as the impedance of a system which when acted on by a force of l dyne, moves with a velocity of 1 cm. per sec.)

Reducing the above expression wig to its absolute value and assuming that a maximum. loss of 3 db referred to. the ideal condition is permissible.

In order to satisfy this equation must be approximately equal to 2 and since X i has already been found to be 4115 ohms, it follows that R must also be about 4115 ohms total resistance (as referred to area A A the area of the acoustic material 5 is 2090 sq. cms. so that the resistance required in the added material is i R A ZXZi=S2 ohms per sq. cm.

=2910 mech. ohms where as the mass reactance of the diaphragm.

at the same frequency, is 21rfm or 3000 mech. ohms. Hence the damping necessary for the purposes of the invention will not unduly reduce the diaphragm motion.

Since the added resistance is equal to the air reactance of 4115 ohms at cycles and is large compared with the radiation resistances r, and n, which are about 66 ohms at 60 cycles, i will be approximately equalto 11,, i. e. substantially the air displaced by the diaphragm will pass through the membrane while the other half will be compressed within the box. While this value may be varied somewhat in any particular case-it is found in general that if more than about of the displaced air passes through the membrane the transient effect is not sufficiently eliminated while if much more than 1/ the displaced air is confined the response is unduly reduced.

The reason for the reduction in cancellation effect obtained according to the invention will be more clearl understood "from the vector diaphragm o vFig. 4 in which i and 11 represent the front and rear waves as in Fig. 3. In the absence of the added resistance R, i will be in phase with z', so that for low frequencies the cancellation effect will be unduly large. However with the resistance I material added the stiffness reactance X, of

. the enclosed air is eifective so that This may be written Hence i is both reduced in magnitude by the ratio indicated and shifted in phase to give a component in phase with i, of only as indicated. In the case chosen for illustrating the invention R=X so that i is reduced to an effective value of one half that for the open-box condition and the cancellation is correspondingly reduced.

The invention has been described with ref erence to a specific structure but it will be understood that it is capable of various modifications within the scope of the following claims.

What is claimed is:

1. The method ofreducing cancellation effects in the air waves set up bydirect radiating diaphragms' which consists in confining a portion of the air displaced by one side of the diaphragm and altering the phase of the motion of that portion of the air not confined.

2. The combination with a sound radiating diaphragm, means for driving the diaphragm and means for baflling the air waves set up by the opposite sides of the diaphragm, of means for partially confining and shifting the phase of the waves set up by one of the sides of the diaphragm. v s

- 3. The combination With a sound radiating dia hragm, means for driving the diaphragm an means for bathing the an waves set up by the opposite sides of the diaphragm of material in the path of the waves set up by one side of the diaphragm offering an impedanceto the waves of substantially pure acoustic resistance.

4. The combination with a sound radiating diaphragm when it is vibrating at low frequencies.

5. The combination with a baflle box-and a loud speaker mounted. in the baffle box of a substantially pure acoustic resistance material covering the normally open side of the box,

6. The combination with a bafiie box and a loud speaker mounted in the loaflle box of an acoustic resistance material covering the normally open side of the box the total resistance of the material being of the same order of magnitude as the elastic reactance of the air in the box at cycles.

7. The combination With an inertia type sound radiating diaphragm and means for driving the diaphragm of substantially pure resistance damping means acoustically coupled to the diaphragm.

8. The combination With a sound radiating dia hragm, means for driving the diaphragm an means for balding the air Waves set up by the opposite sides of the diaphragm of a membrane intercepting the Waves from one side of the diaphragm and having apertures therein of such minuteness that the efiect of the membrane on the Waves is in the nature of a substantiall pure acoustic resistance.

9. The comldination With a sound radiating dia hragm, means for driving the diaphragm and means for bathing the air Waves set up by the opposite sides of the diaphragm. of a member intercepting the Waves from one side of the diaphragm and having an acoustic resistance sufficient to confine not substantially less than one-third nor substantially more than one-half of the air displaced by one side of the diaphragm for important sound frequencies.

In witness whereof we hereunto subscribe our names this 5th da of March, 1930.

PAUL FLANDERS. HENRY C. HARRISON. 

